btrace: support to_stop

btrace: support to_stop

Add support for the to_stop target method to the btrace record target.

gdb/
	* btrace.h (enum btrace_thread_flag) <BTHR_STOP>: New.
	* record-btrace (record_btrace_resume_thread): Clear BTHR_STOP.
	(record_btrace_find_thread_to_move): Also accept threads that have
	BTHR_STOP set.
	(btrace_step_stopped_on_request, record_btrace_stop): New.
	(record_btrace_step_thread): Support BTHR_STOP.
	(record_btrace_wait): Also clear BTHR_STOP when stopping other threads.
	(init_record_btrace_ops): Initialize to_stop.